The Clean Air Act Amendments of 1990 required the EPA to study mercury emissions from combustion and other sources. The results from the coal-fired power plants showed that a certain level of mercury emission control was already achieved by the existing air pollution control devices (APCD). The extent of mercury removal at any given facility depended on a number of factors which included type of coal, fly ash composition, and type of APCD technology. The level of mercury control across the study ranged from 0% to more than 90%. The conclusion was that the existing APCD will not be able to achieve the future mercury emission limits.
One of the most promising solutions for mercury removal from flue gas is Activated Carbon Injection (ACI). Activated carbon is a highly porous, non-toxic, readily available material that has a high affinity for mercury vapor. This technology is already established for use with municipal incinerators. Although the ACI technology is effective for mercury removal, the short contact time between the activated carbon and the flue gas stream results in an inefficient use of the full adsorption capacity of the activated carbon. With coal-fired power plants, this process would also produce a fly ash by-product with an increased carbon content and an increased mercury content. Some of the power plants sell their fly ash by-product to the concrete industry. Fly ash with high carbon and mercury contents can become a waste product with no resale value. A fly ash by-product with high mercury content might also be classified as a hazardous waste and thereby increase disposal costs. Therefore there is a need for a modified ACI technology that is capable of separating the activated carbon from the fly ash. The separated activated carbon can be recycled and reused until it has been fully utilized. The separated fly ash will provide a low-carbon, low mercury fly ash product with maximum resale value or minimal disposal cost.
Mazyck (WO 2004/064078) discusses the use of magnetic powdered activated carbon (MPAC) for removing mercury from the flue gas of a coal-fired power plant or municipal waste incinerator using a powder injection technique. Partially spent MPAC is separated from the fly ash using a rotating magnetic drum separator and then the MPAC is recycled back into the process, but no data are presented as to the efficacy of this technology. The MPAC is an iron impregnated activated carbon that is produced by conventional means. A conventional iron impregnated activated carbon is prepared by first making the activated carbon itself. The activated carbon is loaded or impregnated with a water soluble iron salt, and then treated with KOH to produce a water-insoluble iron hydroxide on the carbon surface. The resulting product is then dried. Production of this carbon for mercury adsorption is expensive and thus impractical to use for removing contaminants from fluid streams.
Oder (U.S. Pat. No. 6,540,088) describes a separation device that uses both electrostatic and magnetic forces to separate particulate mixtures. In one experiment, the author describes the separation of activated carbon from an activated carbon/fly ash mixture, but does not the author describe the use of a magnet adsorbent material.
Chang (U.S. Pat. No. 5,158,580) teaches a two-stage process wherein a majority of fly ash is removed from a flue gas stream using an electrostatic precipitator. The flue gas is then treated using the activated carbon injection technique. The activated carbon and the last traces of the fly are then removed with a bag filter.
Altman (U.S. Ser. No. 2004/0076557) also teaches the treatment of the flue gas in two separate steps. In the first step, an electrostatic precipitator acts as the primary particulate collector. The resulting flue gas stream is then treated with powdered activated carbon. The second process step involves the collection of the activated carbon by a special two stage electrostatic precipitator. The application claims that this technology is cheaper than a second bag house.
Slusarczuk (U.S. Pat. No. 4,201,831) describes a magnetic adsorbent composite made from a mixture of organic material with a ferromagnetic material. The composite is subjected to an activation process. A powdered activated carbon with a reduced settling time is taught, but only for use in liquid systems, more specifically water purification.
Hwang (U.S. Pat. Nos. 6,027,551, 5,227,047, and 5,047,145) describe a froth floatation technology for the separation of unburned carbon from fly ash. The process does not describe the separation of activated carbon from fly ash.
Although technology for the removal of contaminants from a fluid stream using adsorbent materials already exists, there is a need for a simple, effective, and economical technology that allows for the separation of the adsorbent material from other process by-products. Particularly, such a technology is sought for use with by-products in dry processes such as, for example, fly ash and vapor contaminated streams in power plants. Separation of the absorbent material allows for the recycle and full utilization of the adsorbent material, thereby reducing the cost of operation.